When one of your recipes calls for "green chillies", exactly what kind of chili do you mean?
Chillies are not generally known by their names in India. So green chillies are any green chillies that you have. They do of course vary in their heat, so you have to use the amount according to how hot they are and how much heat you like.
As a general rule, the ones that are firm and tightly packed with seeds are the hottest. Shop keeper where you shop, should be able to guide you about the chillies he is selling.